A panmictic population is one where all individuals are potential partners. This assumes that there are no mating restrictions, neither genetic or behavioural, upon the population, and that therefore all recombination is possible. The Wahlund effect assumes that the overall population is panmictic.
In genetics, random mating (or panmixus) involves the mating of individuals regardless of any physical, genetic, or social preference. In other words, the mating between two organisms is not influenced by any environmental, hereditary, or social interaction. Hence, potential mates have an equal chance of being selected. Random mating is a factor assumed in the Hardy-Weinberg principle. Random mating is a separate assumption than natural selection. It is possible to have random mating at the same time the population is undergoing viability selection, for example.
In simpler terms, it is the ability of individuals in a population to move about freely within their habitat, possibly over a range of hundreds to thousands of miles, and thus breed with other members of the population that defines panmixia (or panmicticism).
To signify the importance of this, imagine several different populations of the same species (for example: a grazing herbivore), isolated from each other by some physical characteristic of the environment (dense forest areas separating grazing lands). As time progresses, natural selection, assuming the populations are in differing environments, will slowly move the species toward either separate speciation events or extirpation.
However, if the separating factor is removed before this happens (a road is cut through the forest), and the individuals are allowed to move about freely, the individual populations will still be able to interbreed. As the species populations interbreed over time, they become more uniform, with a decrease in genetic diversity, and thus a decrease in total biodiversity.
Sewall Wright, in attempting to describe the mathematical properties of structured populations, proposed a factor of Panmixia (P) to include in the equations describing the gene frequencies in a population, and accounting for a population's tendency towards panmixia, while a factor of Fixation (F) would account for a population's departure from the Hardy-Weinberg expectation, due to to less than panmictic mating. In this formulation, both quantities are complementary, i.e. P = 1 - F. From this factor of fixation, he later developed the F statistics.